1. Field of the Invention
The present invention relates generally to facsimile transmission apparatus and more particularly to a novel method and apparatus for scanning, storing and transmitting data in a manner which provides accurate reproduction of data transmitted.
2. Description of the Prior Art
In scanning printed or pictorial information for developing data that can be transmitted for facsimile reproduction, finite elemental areas of the scanned surface are normally analyzed and characterized as being either black, white or some level of gray. In those systems wherein levels of gray are not provided for, the scanned elemental areas are usually identified as either a black area or a white area.
In order to improve the transmission and reproduction time of facsimile data, particularly those in which high resolution is not required, it is often desirable to reduce the amount of data transmitted without destroying legibility or other usefulness. However, merely sampling at lower spacial rates destroys image information at the resolution limit. For example, where all areas containing a single bit of data to be reproduced are substantially larger than an elemental scan area, reproduction inaccuracies are rare. However, where areas of white (or black) data equal to or smaller in size than an elemental scan area, or which have a vertical dimension equal to or less than that of a scan line are surrounded by black (or white) data, and a boundary between adjacent scan lines happens to straddle one of these areas, the data representing such areas will often be characterized in a manner which will result in an unaccurate facsimile reproduction.
This problem is illustrated in FIG. 1 in which the shade area 10 represents the black image of a letter "e" including an opening or white area 12 surrounded by the solid or black areas of the letter, the size of the area 12 being small compared to the scan line width W. Where elemental scan areas S are designated as black when 50% or more of the area is black, and are otherwise designated white, areas such as 12 are likely to be straddled by scan lines such that data corresponding to the opening is lost entirely. This result is illustrated by the shaded area 14.
A similar problem arises where a reduction in transmitted data is accomplished by transmitting only every other line of scanned data and with the reproduction equipment repeating each received line twice. As illustrated in FIG. 2, even though the open area 20 in question is larger than a scan element S (but smaller in the vertical dimension than two scan elements) the net result shown at 22 is essentially the same as that shown at 14 in FIG. 1.
One method of reducing the amount of data transmitted is to serially transmit data corresponding to every other element on adjacent scan lines so that the data transmitted corresponds to a zig zag pattern across the document. Another approach is to transmit each line of data in a skipped element pattern. In both cases, simple logic at the reproducing end of the transmission link interpolates the missing elements. These methods produce fairly good results but have two major drawbacks. The first is that if the data is being run length encoded, the number of data transitions is doubled for horizontal edges in the first example and for vertical edges in the second example. Another drawback is that full horizontal and vertical resolution must be printed.